This invention relates to the field of the coating of molded parts, specifically, applying the coatings during the molding process.
Plastics are used widely as enclosures and other mechanical parts because they are inexpensive to make, usually by injection molding and because they possess other desirable physical properties such as light weight, pleasing appearance, sufficient mechanical strength and stiffness, and chemical inertness. However, there are other physical properties that plastics do not normally have that often are required for some particular engineering application. For example, plastic enclosures need to be electrically conductive when there is a requirement for electromagnetic interference shielding. Other applications need hard, abrasion resistant wear surfaces; and yet other plastics require thermal insulation for protection against exposure to high temperatures.
When molded plastic components do need to be modified such as, for example, the addition of an electrically conductive element by thermal spray or painting, it is frequently a secondary operation done more often than not at another location by another manufacturer with a different expertise. This greatly increase the cost of manufacturing the part. Examples of secondary coating operations include sputtering, evaporation, painting, attachment of layers by gluing, screen printing, or thermal spray.
Thermal spray is a versatile and powerful method for applying coatings of metals, ceramics, or cermets to materials in order to modify their physical properties. For example, the surface of a soft aluminum component may be made extremely hard and abrasion resistant by the thermal spray application of tungsten carbide. Or, the surface of an electrically insulating part may be thermally sprayed with copper to make it highly conductive.
Thermal spray technology is a coating technology. It includes systems that use powder as feedstock such as arc plasma, flame spray, and high velocity oxy-fuel (HVOF) systems, or which use wire as feedstock such as arc wire, HVOF wire, and flame spray. Together, these systems comprise a highly versatile technology capable of depositing coatings of nearly any material that melts, be it metal, ceramic, or cermet. They all function by melting the feedstock into tiny droplets, accelerating the droplets in a carrier gas, and depositing the droplets onto a suitably prepared surface. The droplets freeze instantaneously on the surface and build up a thick layer as the thermal spray system is repeatedly traversed across the coated area.
The universal drawback of thermal spray is that a prodigious quantity of heat is involved in the deposition of a coating so that it is very difficult to coat materials that have a low melting point, particularly if they have thin walls. In general, only low melting point materials, such as zinc, tin, or aluminum, can be easily sprayed on low melting point materials, such as plastic. It is very difficult to deposit, for example, aluminum oxide, which is a ceramic with a melting temperature of 205xc2x0 C., onto PC/ABS, which is a plastic that softens below 200xc2x0 C. Thermal spray is most effective when the substrate accepting the coating is made of a material with sufficient mass to absorb the heat and sufficiently high melting point so as not to degrade either itself or the coating.
Therefore, there is a need for improved methods for applying coatings to molded parts.
The invention features methods by which to produce molded plastic or metal parts, which have a desired surface coating that otherwise would require a secondary operation to apply. These methods greatly reduce the manufacturing time and expense because, typically, coatings are applied to molded parts at different facilities using different equipment. In the present methods, a coating, e.g., a high temperature coating, is first applied to a mold and then transferred to the molded part, e.g., at low temperature. The methods described herein greatly enhance the use of low melting point molded parts, such as plastics, by offering the ability to apply engineering coatings, e.g., hard ceramics or high-melting metals, that ordinarily require the parts to be able to withstand high temperatures. In addition, the present methods provide a means of modifying the physical surface properties of a molded part, e.g., giving a very hard surface to a plastic part in order to enhance its engineering properties.
Accordingly, in a first aspect, the invention features a method for fabricating a composite part. The method includes the steps of providing a mold including separable components that define a cavity, wherein the cavity is shaped to form the composite part; separating the components and positioning a mask in front of a surface of the cavity; depositing a coating, e.g., a hard cermet, metal, or ceramic, on the cavity surface, wherein the mask defines an area of the surface to be coated; removing the mask and assembling the components in preparation for molding; and supplying moldable material, e.g., plastic or metal, to the cavity and allowing the material to solidify, wherein when the material solidifies, it adheres to the deposited layer to form the composite part. In various embodiments, the method further includes the step of removing the composite part from the mold. In other embodiments, the coating includes two or more layers of metal, cermet, or ceramic.
An exemplary composite part is a wireless telephone enclosure. In this embodiment, the deposited layer is, for example, an EMI/RFI coating, e.g., one including zinc. In other embodiments, the composite part includes an electrically resistive heater. Desirably, the electrically resistive heater includes an electrically insulating layer, e.g., one containing aluminum oxide, and a layer having a formulated resistivity, e.g., one containing molybdenum silicide. In other embodiments, the heater includes nickel-chrome.
The foregoing and other objects, features, and advantages of the invention will be apparent from the following detailed description of the invention and the accompanying drawings.